The present invention relates generally to solid plastic foams, to articles of manufacture produced from such foams, and to methods of producing such foams and articles and relates more particularly to an improved foam made of poly(4-methyl-1-pentene), to articles of manufacture produced therefrom, and to methods of producing the foam and the articles of manufacture.
In laser fusion studies, there has been a need for multishell fusion targets in which the concentricity between the inner and outer shells does not vary over a 2% range. Also needed between the two shells was a cushion layer having low atomic number (i.e., low-Z). For some studies in inertial confinement fusion (i.e., ICF), it was desired that the cushion layer be formed from small cell plastic foam having low-Z, low density, and cell size of 10 to 30 .mu.m and being made of a material containing only carbon and hydrogen.
Because poly(4-methyl-1-pentene) contains only carbon and hydrogen and has a lower bulk density (and hence will form low density foam more readily) than other hydrocarbon polymers (for example, polyethylene, polystyrene, etc.) low density microcellular foams of poly(4-methyl-1-pentene) were desired for ICF targets; however, it was also a requirement that the foam be machined into structures to tolerances of 0.0001 inch (without any defects larger than 0.0001 inch).
In the prior art, many patents have disclosed forming foams of poly(4-methyl-1-pentene).
In U.S. Pat. No. 3,378,507 to Sargent et al., "Producing Microporous Polymers," a process for producing solid, microporous hydrocarbon polymers (including 4-methyl-1-pentene) and products are disclosed. Water-soluble, anionic surfactant (which is preferably a solid) is incorporated into solid, thermoplastic polymer, thus forming a porous structure. Thereafter, the surfactant is removed from the structure. However, due to the use in Sargent et al. of no more than 90% by weight of surfactant in the total weight of surfactant and hydrocarbon polymers, the density of the resulting structures will be higher than was required for ICF targets; and in Sargent et al. there was no disclosure of machinable microcellular foam having a density within the range from about 10 to about 100 mg/cc. No incentive was there provided for forming a very low density structure. Instead, the patent appeared to teach away from such a structure by the statement (at col. 6) that if the amount of surfactant used is greater than 90%, the hydrocarbon structure left after extraction is extremely porous and too weak to be of any utility.
In U.S. Pat. No. 4,247,498 to Castro, poly(4-methyl-1-pentene) is disclosed as one of many polymers which can be used to form microporous polymer products. With the polymer is mixed a compatible material which is generally a liquid at ambient temperatures. However, as disclosed in Castro, materials which are solid at room temperature may be employed, so long as they can form solutions with the polymer at elevated temperatures and so long as they do not interfere with the formation of the microporous structure. More specifically, a solid material may be used, provided that phase separation occurs by liquid-liquid separation rather than liquid-solid separation during the cooling step. In examples 244 and 255 of Castro, methylpentene (which is believed to be the same material as poly(4-methyl-1-pentene) was used in the standard preparation procedure of Castro. In Table X of Castro, twelve compatible materials are given for the polymer, one of which (naphthalene) was a solid at room temperature. However, the Castro patent clearly emphasizes using a liquid, and no incentive is provided for using the particular material which is used in the method of the present invention (described below).
Following Castro's method, one obtains plastic foam objects by allowing the solution of polymer and compatible liquid at an elevated temperature "to assume a desired shape" and "cooling said solution in said desired shape at a rate and to a temperature sufficient to initiate thermodynamic, non-equilibrium liquid-liquid phase separation" (as claimed in claim 1 of Castro and as disclosed in the specification at column 15 at lines 26-36, where further processing of blocks of the microporous material includes conventional extrusion, injection molding or other related techniques). However, in cooling the polymer within a mold, the solution generally will shrink in size at a rate different from that of the mold due to differences in the coefficients of thermal expansion of the polymer solution and the mold.
Therefore, by following the methods disclosed in Castro, one cannot produce final foam objects to very tight tolerances; and objects of only rough final dimensions will be obtained. Furthermore, the foams which are formed by the process of Castro are very delicate; and, thus, any fabrication method which involves mechanical contact with the foam usually has distorted, damaged, or destroyed the foam structure.
In U.S. Pat. No. 2,671,953 to Balke, "Metal Body of High Porosity," a porous metallic body was impregnated with a wax; and then the body was sawed into blanks and turned in a lathe. Thereafter, the wax was volatilized from the metal body. This patent, however, did not address any materials other than metals.
Therefore, despite what has been known in the prior art, a need has existed for low density, microcellular articles of manufacture consisting of poly(4-methyl-1-pentene) and machined to tolerances of 0.0001 inch.